S134
Brachytherapy - Physics
ESTRO 2026
Purpose/Objective: In high-dose-rate brachytherapy for locally advanced cervical cancer, dose accumulation is typically performed by summing fraction DVH parameters in EQD2. However, anatomical changes between treatments challenge the validity of this approach [1]. Deformable image registration (DIR) offers a potential solution, yet its application in MR-to-MR multi-fraction dose accumulation is not established [2]. The anatomically constrained deformation algorithm (ANACONDA; RaySearch Laboratories AB, Stockholm, Sweden) has not been validated for this setting, and quantitative metrics tailored to brachytherapy DIR are lacking [3,4]. This study investigates the feasibility of ANACONDA DIR voxel-based dose accumulation (DIR- DA) in multi-fraction cervical brachytherapy and surface-based metrics for DIR validation. Material/Methods: 25 patients with 3 applicator insertions in 3 weeks – totalling 75 planned fractions – had MRI-only planning using a standardised 3D T2-weighted spin-echo protocol on a 1.5T Philips Ingenia simulator. 52% of cases involved interstitial needles.50 DIRs were performed using ANACONDA; controlling structures were bladder, rectum, sigmoid colon, small bowel, uterus, and applicator. DIR acceptability for dose accumulation was visually scored on a 3-point Likert scale (1=acceptable, 2=borderline, 3=unacceptable) by a trained physicist. Quantitative assessment included volumetric and surface dice similarity coefficients (DSC), and a surface-gamma analysis incorporating distance-to-agreement and point-dose difference [3]. Metric correlation with qualitative scores was evaluated using Spearman’s Rank.Total D2cc in EQD2 ( α / β =3) was calculated per organ-at-risk using DIR-DA and compared to DVH parameter addition. Results: Visual assessment determined DIRs were acceptable for 54% of bladder, 52% of rectum, 40% of sigmoid, and 9% of small bowel contours; borderline in 42%, 44%, 34%, and 59%, respectively (table 1).Differences in total D2cc between DIR-DA and DVH parameter addition were <2Gy EQD2 in most cases with DIR scored as acceptable (table 1).
Conclusion: Correction factors derived from MC simulations for a phantom designed for an ETE dosimetry audit in gynaecological brachytherapy have been experimentally validated paving the way for a future international multi-centre pilot study for broader validation. Keywords: dosimetry audit, Monte Carlo, film dosimetry References: 1. Famulari G, Renaud M-A, Poole CM, Evans MDC, Seuntjens J, Enger SA RapidBrachyMCTPS: A Monte Carlo-based treatment planning system for brachytherapy applications, Phys Med Biol. 2018, 63(17):1750007, doi: 10.1088/1361-6560/aad97a. PMID: 300950772. Chelminski K, Dimitriadis A, Abdulrahim R, Kazantsev P, Granizo-Roman E, Kalinowski J, Abbasi Enger S, Azangwe G, Carrara M, Swamidas J. Monte Carlo simulated correction factors for high dose rate brachytherapy postal dosimetry audit methodology. Phys Imaging Radiat Oncol. 2024 Oct 22; 32:100657, doi: 10.1016/j.phro.2024.100657, PMID: 39534277 Digital Poster Highlight 3112 Clinical acceptability of ANACONDA DIR for MR-to- MR deformable dose accumulation in cervix brachytherapy and the role of surface-based metrics Anna K Clark 1,2 , Bashar Al-Qaisieh 1 , David Bird 1 , Kari Tanderup 3,4 , Peter J Hoskin 5,6 , Ann M Henry 2,7 , Caroline L Jones 1 , Peter Bownes 1 1 Department of Medical Physics, Leeds Cancer Centre, St James's University Hospital, Leeds, United Kingdom. 2 Leeds Institute of Medical Research, University of Leeds, Leeds, United Kingdom. 3 Danish Center for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark. 4 Department of Clinical Medicine, Aarhus University, Aarhus, Denmark. 5 Department of Radiation Oncology, Mount Vernon Cancer Centre, Northwood, United Kingdom. 6 Division of Cancer Sciences, University of Manchester, Manchester, United Kingdom. 7 Department of Clinical Oncology, Leeds Cancer Centre, St James's University Hospital, Leeds, United Kingdom
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